Preconcentration is a well known technique in analytical chemistry and is used in the detection of trace contaminants in both gases and liquids. In the analysis of a gas, the gas, commonly at or near atmospheric pressure, is drawn through or past a sorbent that selectively adsorbs or traps the trace constituents. The adsorption usually takes place on the surface of a solid sorbent which in most cases is a powder, "wool" or sheet. Sorbents are selected on the basis of their ability to adsorb compounds of interest and their adsorbing capacity. The amount of trace constituent that is adsorbed is proportional to the time that the gas is drawn through or otherwise contacts the sorbent ("sampling time"), before the sorbent is saturated. Following the time period wherein the sampling takes place, the sorbent is induced to release the trace constituents, usually accomplished by rapidly heating the sorbent. The temperature of the sorbent may be raised sufficiently in one step so that all of the analytes are desorbed simultaneously ("flash desorption"), or it may be increased more slowly so that different chemical species, which desorb at different temperatures, are released at different times ("temperature programmed desorption"). In either case, the trace constituent is re-introduced into a carrier gas flow in a desorption interval of time that is much shorter than the period of time wherein the sampling was accomplished and, as a result, the concentration of the plug of analyte in the carrier gas is higher than was its concentration in the original sample gas. An example of a device that so operates is the Universal Automated Concentrator manufactured by Envirochem, Kemblesville, Pa.
It is common for the sorbent to be in the form of a powder confined within a thin-walled tube which is sufficiently granular for the gas to flow through it, and to provide that the flow of carrier gas through the sorbent during the desorption period is in an opposite direction to the flow through it during the sampling period. An example of a preconcentrator which employs this method is the Automatic Chemical Agent Monitor System (ACAMS), which was developed by Southern Research Institute for the U.S. Army and which is manufactured by ABB Process Analytics (formerly Combustion Engineering, Inc.), of Lewisburg, W. Va., and CMS Research Corporation of Birmingham, Ala.
Preconcentrators also are used in the analysis of volatile compounds in liquids, typically water, by using "purge and trap" techniques. For this, an inert gas is bubbled through the liquid to entrain the volatile contaminants and carry them to a sorbent similar to the ones described above for gas samples. After the sampling (purging and trapping) is complete, the analytes are desorbed from the sorbent, as described above. The Tekmar LSC 2000, manufactured by Tekmar Company of Cincinnati, Ohio, is an example of a commercially available "purge and trap" instrument.
Preconcentrators are commonly used in conjunction with gas chromatographs (GC) to separate and identify the analytes. The flow rate of the carrier gas through the preconcentrator (or the final trap) during the desorption phase, in general, is determined by the characteristics of the GC column. Capillary columns, for example, use lower mass flow rates than traditional packed columns. In addition, to increase the resolution and sensitivity of the GC, the desorbed analytes are often retrapped, usually by cryogenic means, at or near the entrance of the CG column, from which they are desorbed rapidly to ensure that the sample is injected into the column in a short period of time. The Tekmar LSC 2000 "purge and trap" device is used with commercially available gas chromatographs using packed and megabore columns, and the Tekmar Model 1000 capillary interface, which employs cryogenic refocusing, allows the Tekmar LSC 2 "purge and trap" device to operate with capillary column gas chromatographs.
The following Publications are cited for more comprehensive explanations of prior art apparatus and techniques mentioned above, and are incorporated herein by reference:
1. SW-846, "Test Methods for Evaluation of Solid Waste," Third Edition, EPA Publication OSW 0000846; Methods 0030 and 5040. PA0 2. D. L. Fox, Anal. Chem. 61. IR (1989), and references within. PA0 3. D. L. Fox, Anal. Chem. 59, 280R (1987), and references within. PA0 4. R. A. Kagel and S. O. Farwell, Anal. Chem. 58. 1197 (1986). PA0 5. P. A. Steudler and W. Kijowski, Anal. Chem. 56, 1432 (1984). PA0 6. R. Otson, J. M. Leach, and L. T. K. Chung, Anal. Chem. 59. 58 (1987). PA0 7. T. Bellar and J. Lichtenberg, J. Am. Water Works Assoc. 66, 739 (1974). 8. United States Environmental Protection Agency, "Method 524.2. Volatile Organic Compounds in Water by Purge and Trap Capillary Column Gas Chromatography/Mass Spectrometry," August, 1986. PA0 9. N. Kirshen, Am. Lab. 16 (12), 60 (1984). PA0 10. L. D. Landau and E. M. Lifshitz, "Fluid Mechanics," Translated by J. B. Sykes and W. H. Reid, Pergamon Press, London, 1959, p. 59.